1. Field of the Invention
The present invention relates generally to an uplink (UL) power control apparatus and method in a broadband wireless communication system, and in particular, to an apparatus and method for stably switching a UL power control mode in the broadband wireless communication system.
2. Description of the Related Art
Research to provide users with varying Quality of Service (QoS) at a high data rate is an objective of the fourth generation (4G) communication systems. Specifically, research into the high rate support service to guarantee mobility and QoS in the 4G communication systems, such as wireless Local Area Networks (LAN) and wireless Metropolitan Area Networks (MAN) guaranteeing relatively high data rates, has been under way.
The Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system adopts an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support the broadband transmission network in physical channels of the wireless MAN system. Applying the OFDM/OFDMA scheme to the wireless MAN system, the IEEE 802.16 communication system enables the high data transmission by transmitting the physical channel signal using a plurality of subcarriers.
In the OFDMA communication system, uplink (UL) signals may act as excessive interference to other Mobile Stations (MSs) or neighbor cells according to the transmit power, or cause a decrease in the Base Station (BS) reception power. Thus, an appropriate power control is required according to the required Carrier to Interference and Noise Ratio (CINR).
Typically, the power control mode can be broadly classified into a closed loop power control and an open loop power control.
The closed loop power control compensates for the UL transmit power of the MS under the control of the BS. However, the closed loop power control may degrade the accuracy of the power control in the packet communication system. The BS determines the power control range using the CINR value of the packets received in the UL. Even when the BS occasionally receives the packets and performs the power control for every packet reception, the accuracy of the power control may degrade because there arises a difference between the time of the UL packet transmission from the MS and the time of the power control direction from the BS.
In the open loop power control, on the assumption that the path loss of the uplink is equal to the path loss of the downlink (DL), the MS adjusts the transmit power of the UL signal by estimating the DL path loss by itself. That is, the MS adjusts the UL power using the required CINR received from the BS, interference and noise level information of the uplink, and the DL path loss. The BS can additionally direct the fine adjustment to the MS based on the CINR value of the received packets.
As discussed above, the open loop power control can greatly improve the accuracy of the power control thanks to the MS's own transmit power adjustment and the BS's additional power adjustment direction, compared to the closed loop power control.
Therefore, the MS in the OFDMA communication system performs the closed loop power control in the initial network entry phase, and then performs the open loop power control by switching the power control mode.
FIG. 1 depicts a conventional switching procedure from the closed loop power control to the open loop power control in a broadband wireless communication system.
At the initial access, MS 10 of FIG. 1 receives Downlink Channel Description/Uplink Channel Description (DCD/UCD) messages from BS 20 in step 101, and acquires information (parameters) required for the initial access from the received messages. In doing so, the MS 10 can acquire parameters relating to the initial ranging. The MS 10 receives UL interference and noise level information from the BS 20 in step 103.
In step 105, the MS 10 sets an initial transmit power based on the information received from the BS 20. In step 107, the MS 10 sends an initial ranging code to the BS 20 with the initial transmit power. The BS 20 sends a ranging response RNG_RSP message to the MS 10 in reply to the initial ranging code in step 109. The BS 20 sends a band allocation message CDMA Alloc IE for the ranging request to the MS 10 in step 111.
When the RNG_RSP message is not received within a present time after the initial ranging code is transmitted, the MS 10 retransmits the initial ranging code at an increased transmit power level. When the RNG_RSP message is received within the preset time, the MS 10 performs Network Entry (N/E) procedures starting from the initial ranging in step 113. The N/E procedures include the initial ranging RNG_REQ/RSP, the basic capability negotiation SBC_REQ/RSP, and the authentication PKM_REQ/RSP.
After the N/E procedures, the MS 10 enters the closed loop power control mode which adjusts the transmit power according to power control IE from the BS 20 in step 115.
The BS 20 checks the open loop power control capability of the MS 10 through the basic capability negotiation procedure. After the N/E procedures, the BS 20 requests the MS 10 to switch to the open loop power control mode by sending Power control Mode Change ReSPonse (PMC_RSP) message in step 117. The MS 10 transmits Power control Mode Change REQuest (PMC_REQ) message to the BS 20 in response in step 119, and changes the power control mode to the open loop power control in step 121.
The MS 10 calculates the transmit power P according to the open loop power control based on Equation (1).
                    P        =                  L          +                      C            /            N                    +          NI          -                      10            ⁢                                                  ⁢                                          log                10                            ⁡                              (                R                )                                              +                      Offset_SS            perSS                    +                      Offset_BS            perSS                                              (        1        )            
Parameters in Equation (1) are defined as below:                P: transmit power (dBm) per subcarrier of UL burst.        L: average estimation value for propagation path loss which is calculated using the total receive power measured through active subcarriers of the preamble and Efficient Isotropic Radiation Power parameter from the BS (BS_EIRP). BS_EIRP parameter indicative of the BS transmit power is received using a DCD message.        C/N: received CINR value required by Modulation and Coding Scheme (MCS) level of the UL burst.        NI: estimation value of the average interference and noise power (dBm) per subcarrier measured at the BS, which is provided to every MS as common information.        R: number of repetitions according to the MCS level        Offset_SSperSS: MS power compensation value controlled by the MS, which is always zero in the passive open loop power control mode.        Offset_BSperSS: BS power compensation value controlled by the BS. When this value is set using PMC_RSP message, Offset_BSperSS is substituted with the value of PMC_RSP message. When the BS directs the power fine adjustment using the power control IE, the accumulated power adjustment values in the power control IE are used as Offset_BSperSS value. Alternatively, the accumulation of the power adjustment values in RNG_RSP message received from the BS can be used as Offset_BSperSS value.        
The conventional art has a number of drawbacks.
When a Relay Station (RS) (or Repeater) is installed to the system, and particularly when the transmit power of the RS downlink (RS→MS) differs from the transmit power of the RS uplink (RS→BS), the open loop power control may not normally work. This is because the open loop power control fundamentally assumes that the DL propagation path loss is equal to the UL propagation path loss.
If the RS DL transmit power is greater than the UL transmit power, the MS estimates the UL propagation path loss by measuring the DL propagation path loss. Accordingly, the estimated UL propagation path loss is less than the actual path loss. In this case, when the open loop power control is conducted based on Equation (1), the MS transmits the UL packet with much less power than the required transmit power. As a result, the BS may not receive the UL signal or the error rate of the UL packet may increase. Particularly, when the power control mode is changed (closed loop power control→open loop power control) with the different propagation path losses between the downlink and the uplink, its influence is considerable.
If the set BS_EIRP value of DCD is greater than the actual BS output value, then the MS calculates the DL path loss greater than the actual value. In this case, since the unnecessarily large UL transmit output may be set in the open loop power control, this can exert influence on other MS signals in view of the BS reception. In other words, when the BS performs an Automatic Gain Control (AGC) before a Fast Fourier Transform (FFT) stage, the AGC operates based on the summation of all UL signal powers. Thus, this may act as the interference to the MS signal having the relatively weak receive signal.
By contrast, when the set BS_EIRP value of DCD is less than the actual BS output value, the calculated MS DL path loss becomes less than the actual value. In this case, since the less UL transmit output than is necessary can be set in the open loop power control, the BS may not receive the MS signal.
As discussed above, since the open loop power control can compute the improper transmit power because of the various external factors, what is needed is a method for stably accomplishing the open loop power control. Furthermore, such errors are notable when the closed loop power control is changed to the open loop power control. Therefore, it is required to properly maintain the transmit power when the power control mode is changed.